The present invention relates to decontamination of fluids containing ionic contaminants, especially water containing anionic contaminants, such as arsenic and chromium in the form of arsenate, arsenite, and chromate. In particular, the present invention relates to an improved water decontamination process comprising contacting water containing anionic contaminants with an enhanced coagulant that binds anionic species predominantly through the formation of surface complexes, wherein the sorbent material comprises a trivalent metal cation-based coagulant mixed with a divalent metal cation modifier.
For purposes of this disclosure, unless otherwise specified, the term “metal oxides” is intended to include both metal oxides and metal hydroxides. Likewise, the term “metal sulfates” is intended to include both metal sulfates and hydrous metal sulfates. Similarly, for purposes of this disclosure, “arsenic contaminants” includes arsenates, AsV, and arsenites, AsIII.
Arsenic contaminants are examples of anionic contaminants that may be present in water as a result of natural as well as human-mediated causes. The long-term availability of safe and affordable drinking water depends, in part, on availability of effective and economical treatment means for removing arsenic contaminants (as well as other anionic contaminants, including chromate) from water. Successful treatment strategies, in turn depend on not otherwise significantly altering the water characteristics (for example, its pH) in ways that would make it non-potable.
Arsenic and other anionic contaminants likewise pose risks when present in fluids other than drinking water sources. For example, wastewater streams often contain such contaminants and require remediation even where they are not considered to be directly associated with potable drinking water sources. Inorganic arsenic in groundwater usually exists as a combination of neutral AsIII (arsenite) and anionic AsV (arsenate). Arsenite is believed to be more toxic than arsenate. Conventional treatment methods generally remove arsenate more efficiently than arsenite because of coulombic attraction. Fortunately, AsIII (arsenite) is easily oxidized to AsV (arsenate) by chlorine, permanganate, ozone, chlorine bleach, and peroxides (some of which are used in conventional water treatment systems). However, AsIII, (arsenite) is not easily oxidized by chlorine dioxide (ClO2), preformed chloramines, oxygen, or UV light.
Various sorbent methods for removing arsenic contaminants and other anionic contaminants from water have been used and developed previously. For example, certain trivalent metal oxide compounds, such as Al2O3 and Fe2O3 have been demonstrated to sorb anionic contaminants, including arsenic contaminants, from water. A drawback associated with use of such trivalent compounds alone is that, because they typically exhibit a point of zero charge from pH 7 to 9, the water to be treated may need to be acidified in order for these compounds to sorb anions to a significant degree. Thus, after treatment, in order to restore the potability of the treated water, further amendments must be added to bring the pH back up to a safely drinkable range. Similarly, tetravalent metal oxides such as SiO2 could be effective anion sorbents, however, their point of zero charge is typically around pH 2, so extremely acidic conditions would needed for tetravalent metal oxides to sorb anions. Additionally, these substances are considered likely to fall outside of the range of useful sorbents because of other chemical issues associated with operating at such low pH.
The divalent metal oxide MgO or hydroxide, Mg(OH)2, likewise, has been shown chemically to sorb anions including arsenic in water. Although use of MgO does not necessitate driving the pH of water outside of the potable range (divalent metal oxides tend to exhibit a point of zero charge that is pH 10 or higher), the long-term effectiveness of MgO as a sorbent for water decontamination, however, can be limited. This is due to its tendency to form carbonates in the presence of carbon in the water from natural (e.g., biological and atmospheric) sources. When this occurs, the carbonate species formed at the surface lack any significant electrostatic attraction for negatively charged ions. Thus, the sorbency of the MgO can be short-lived, absent taking steps to reverse the carbonate reaction and restore the sorbent.
The sorbency methods just discussed rely on the electrostatic attraction between positively charged surface species and negatively charged (i.e., anionic) contaminants. An altogether different mechanism that has been exploited to decontaminate water containing ionic contaminant species is ion exchange. Examples of ion exchange materials suitable for water decontamination include hydrotalcites (which exchange anions) and zeolites (which exchange cations). Although ion exchange materials have been shown to be effective without causing the types of problems associated with Fe2O3 and Al2O3 (pH concerns) or MgO (carbonate issues), ion exchange materials can be very expensive. Zeolites that allow for separations based on size are also used in some decontamination applications, but they do not sorb anionic species such as chromate and arsenic contaminants in water.
Coagulation is a commonly used method for treating water. Existing coagulation approaches for treating drinking water, wastewater, and surface water rely traditionally on adding trivalent metal cation coagulants (e.g., based on Fe(III) or Al(III)) to the contaminated water. The principal use of coagulants is to destabilize particulate suspensions and to enhance the rate of floc formation. Examples of these trivalent cation metal coagulants include hydrolyzable metal salts (HMS), such as iron-based salts (e.g., ferric chloride, ferric sulfate, and ferrous sulfate) and aluminum-based salts (e.g., aluminum sulfate, alum, and sodium aluminate (NaAlO2)). The hydrolyzable metal salt coagulant (e.g., ferric chloride or alum) is added to the water containing anionic contaminants (and sometimes containing organic matter). The pH is then adjusted to near the pH of minimum solubility of, respectively, ferric hydroxide Fe(OH)3 or aluminum hydroxide Al(OH)3 (pH˜7–8), whereupon the salts hydrolyze to form a series of iron or aluminum hydroxide precipitates (i.e., hydrolysis products), generically called flocs. The most commonly used coagulants in industry are ferric chloride and alum. Typically, they are supplied from the manufacturer in a highly concentrated form, and are injected into the flowstream of a water treatment plant upstream of a mixing or flocculation chamber.
The flocs formed by HMS coagulants adsorb natural organic materials (NOMs), and certain inorganic materials, such as phosphates, arsenic compounds, fluoride, selenate, and borate. Negatively charged anions, including AsV, subsequently sorb from solution to bind with positively charged surface groups on the floc. The anion contaminant (e.g., arsenate) is sorbed as a tightly bound complex on the floc, which is removed from solution when the floc is separated from the water (typically in a clarifier or through filtration), leaving purified water. When an aluminum hydroxide floc is used, the sorbing process is most effective at a pH of 6.0–6.5. In general, iron-based flocs are significantly more effective at removing arsenic than aluminum-based flocs.
The phrases “hydrolyzable metal salts” and “metal salt coagulants” are defined herein to include prehydrolyzed metal salt solutions, such as polyaluminum chloride or polyaluminum hydroxychloride (PACl), and polyiron chloride (PlCl); as well as solutions that contain a strong acid, typically sulfuric acid (e.g., acidulated alum or acid alum).
Special additives (e.g., phosphoric acid, sodium silicate, and calcium salts) can be added to solutions of metal salt coagulants to improve performance or address specific problems. Metal salt solutions are also sold premixed with cationic polyelectrolyte coagulant compounds, such as epichlorohydrin dimethylamine (epiDMA) and polydiallyl dimethylammonium chloride (polyuDADMAC), which belong to a class called quaternary amines.
Polyelectrolytes can also be used to treat water. They are often categorized as either primary coagulant polymers or flocculent polymers (i.e., coagulation aid). The primary coagulant polymers can be used alone, or in combination with, metal salt coagulants, such as alum. Additionally, the flocculent polymer coagulant aids are often added after flocculation to increase the size and strength of particle aggregates (i.e., flocs). Activated silica (SiO2) can also be used as a coagulation aid in combination with alum to improve the efficiency of flocculation and sedimentation, especially in cold winter months.
The costs of using a coagulation process for water treatment include the supply costs of the coagulant medium; downtime due to backwash and filter replacement; and the cost of replacing clogged filters. As the regulations for water quality become tighter, as has recently happened for arsenic levels in water (i.e., from 50 ppb to 10 ppb), the costs of treating water can greatly increase, and can become prohibitively high for small community water systems and individual wells.
The need remains, therefore, for an improved water decontamination process that is more effective and less expensive than existing coagulation methods for removing anionic contaminants, including chromate, arsenates and arsenites.